Nomenclature and Identity
Group name: The elements found in Group 17 of the periodic table are known as halogens. The word “halogen” means “salt-former,” because these elements often form salts when they react with metals.
Member elements: This group is made up of five well-known elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At). Each of these has its own color, state, and special properties, but they all belong to the same chemical family.
Electronic Configuration
Seven valence electrons: All halogen atoms have seven electrons in their outermost energy level. Since atoms are most stable when their outer shell is full, halogens only need to gain one more electron to reach this stable state. This is why they are so reactive.
Molecular Structure
Diatomic nature: In nature, halogen elements don’t usually exist as single atoms. Instead, they pair up to form molecules with two atoms, like F₂ or Cl₂. This is because they are more stable when bonded together.
Chemical Reactivity
Highly reactive: Halogens are among the most reactive non-metal elements. They react quickly and easily with metals and hydrogen to form compounds, especially because they want to gain that one extra electron.
Reactivity trend: As you move down the group from fluorine to astatine, each element becomes a little less reactive. This is because larger atoms hold onto electrons more loosely, making them less eager to grab new ones.
Physical Properties
Low melting/boiling points: Compared to metals and many other elements, halogens have relatively low temperatures at which they melt and boil, which means they easily change states between solid, liquid, and gas.
Increasing phase points: As you go from the top to the bottom of the group, each element has a higher melting and boiling point. This means iodine and astatine need more heat to melt or boil than fluorine or chlorine.
States at room temperature: The physical state of halogens at room temperature varies. Fluorine and chlorine are gases, bromine is a liquid, and iodine and astatine are solids. This shows how their properties gradually change down the group.
Poor conductors: Halogens are poor conductors of heat and electricity, just like many non-metal elements. They do not let electricity or heat pass through them easily.
Distinct colors: Each halogen has its own unique color. For example, fluorine is pale yellow, chlorine is greenish-yellow, bromine is a reddish-brown liquid, and iodine is black but turns into a purple vapor when heated. These colors help scientists identify them.
Atomic size trend: As you go down the group, each element has more electron shells, making its atoms larger in size. This also affects how they react with other substances.
Chemical Properties
Formation of halides: Halogens easily form compounds called halides when they react with metals or hydrogen. For example, when chlorine reacts with sodium, it forms sodium chloride—common table salt.
Electron gain: Because halogens have seven electrons in their outer shell, they quickly accept one more electron to make a total of eight, which is a stable configuration. When this happens, they form negatively charged ions called anions, specifically with a -1 charge.
Reactions with Metals, Hydrogen, and Alkali Metals
Metal halides: When halogens react with metals, they create metal halides—salts that are made of a metal and a halogen. These are often white solids and dissolve easily in water.
Hydrogen halides: Halogens can also react with hydrogen to form hydrogen halides, such as hydrogen chloride (HCl). These can dissolve in water to form acids like hydrochloric acid.
Reactions with alkali metals: Halogens react very strongly with alkali metals like sodium or potassium. These reactions are fast and sometimes explosive, and they produce metal halides.
Exothermic reactions: When halogens react with other substances, they often release a lot of heat. These types of reactions are called exothermic because they give off energy in the form of heat.
Trends in Properties Down the Group
Reactivity trend: The reactivity of halogens goes down as you move from fluorine to astatine. This happens because larger atoms are less able to attract electrons from other atoms.
Atomic size trend: As more electron shells are added moving down the group, the size of the atoms increases. This is a common trend in the periodic table.
Melting/boiling trend: Both melting and boiling points increase down the group, which means the lower halogens need more heat to change their state.
Color intensity: The colors of halogens get stronger and darker as you move from fluorine to iodine and astatine. This trend in color is one of the easiest ways to tell them apart.